This invention relates generally to ghost cancellation circuits for eliminating ghost interferences in television systems and more particularly to such a ghost cancellation circuit which operates at a very high speed.
There has been proposed a ghost cancellation circuit which cancels ghosts in a received television signal with the aid of a ghost cancellation reference signal having a waveform of sin x/x at its leading edge (hereinafter referred to as "GCR signal") to thereby obtain a high picture quality.
A ghost cancellation circuit of this kind generally comprises a finite impulse response (FIR) filter (or a non-recursive filter) for mainly canceling nearby ghosts and an infinite impulse response (IIR) filter (or a recursive filter) for mainly canceling normal ghosts.
Conventionally, with the ghost cancellation circuit having such a structure, respective filter coefficients for an FIR filter and an IIR filter are calculated by means of a microprocessor or the like based on a received GCR signal, which may temporarily be stored in a memory, and a GCR signal previously stored in its circuit. The filter coefficients (i.e., the coefficients for filter taps) are then set to the filters, whereafter a received television signal is passed through the filters to thereby cancel its ghosts. In this case, the filter coefficients for the FIR filter can be calculated based on the received television signal in the following manner. First, the frequency characteristic of the received GCR signal is calculated by means, for example, of the fast Fourier transformation (FFT). The frequency characteristic of the known GCR signal is then divided by the above calculated frequency characteristic, and the result of this division is subjected to an inverse FFT to thereby obtain the filter coefficients. Alternatively, the least square method may be used. More specifically, the filter coefficients are calculated by means of the least square method based on the waveform of the received GCR signal and that of the known GCR signal. Filter coefficients for the IIR filter can also be calculated in a manner described above for the FIR filter. In order to achieve a sufficient ghost cancellation, however, several tens of filter coefficients must be calculated for the FIR filter and hundreds of filter coefficients for the IIR filter. Although the calculation of the filter coefficients for the FIR filter can be done in a relatively short time, that for the IIR filter takes a substantial time, which has been a bar against the fast operation of this kind of ghost cancellation circuit.
In the above calculation, if the waveform of the received GCR signal contains high-frequency noises, the filter coefficients for the FIR and IIR filters calculated based on this received GCR signal become inaccurate, which results in an insufficient cancellation of ghosts.
It is therefore an object of the invention to provide a ghost cancellation circuit comprising an FIR filter and an IIR filter which operates at a higher speed, that is to say, a ghost cancellation circuit in which the filter coefficients can be set to these filters in a reduced time.
It is another object of the invention to provide a ghost cancellation circuit which can cancel ghosts in the received television signal more accurately.